In certain radiofrequency data transmitting applications, it has been observed that the transmitting or receiving antenna could have an impedance strongly dependent on conditions external to the antenna, and notably dependent on the environment in which the antenna is placed.
For example, in medical telemetry, it can happen that the antenna is inserted in a probe placed in a human body and the impedance then strongly depends on the biological environment in which the antenna is located. It depends on electrical properties (conductivity, dielectric constant) of the surrounding tissues (muscles, fat) or of the liquid environment (blood, other liquids) in which the antenna can be immersed.
Even in more conventional radiofrequency transmission applications (mobile telephony, etc.) the impedance of the antenna can vary as a function of the environment in which the user is located.
A transmitting (or respectively receiving) system comprises at least one amplifier with which can be associated one or more filters and an antenna (at the output of the amplifier for a transmitting system and at the input of the amplifier for a receiving system).
In general, the antenna impedance variations are particularly sensitive for very small antennas having a high quality factor, used in applications having severe miniaturization constraints.
These impedance variations can result in losses called mismatch losses: these losses result from the fact that the transmitting system which feeds the antenna, or the receiving system which receives a signal from the antenna, is generally designed to have optimum performance when it is loaded (at the output for the transmitting system or at the input for the receiving system) by a well-defined nominal impedance; it has degraded performance when it is loaded by an impedance different from its nominal value. The mismatch losses can be as high as 40 dB.
That is why it has already been tried to interpose, between the output of the amplifier and the transmitting antenna (and this could also be done at the input for a receiving antenna), an impedance matching network, which means that the amplifier sees an impedance different from that of the antenna and preferably equal to the nominal value for which it was designed, for example 100 ohms or 500 ohms. The matching network can be tuned, that is to say its capacitive and/or inductive components have variable values to take account of the environmental conditions of the antenna such that the matching is the best possible whatever the circumstances may be.
In Patent Application US 2009-0066440, there was proposed an automatic impedance matching method in a transmitting or receiving system, in which both the amplitude and the phase of the current and of the voltage at the output of the transmitting system (or at the input of the receiving system) are detected. The ratio between the voltage and the current is representative of the load impedance Zm seen by the system loaded with the assembly formed by the matching network and the antenna of impedance Zant. The load impedance Zm is measured and the antenna impedance Zant is computed from the measured load impedance Zm and the impedances of the matching network whose configuration at the time of measurement is known, and finally the modification which must be applied to one or more of the impedances of the matching network is computed in order to result in the impedance seen by the amplifier becoming matched to the nominal impedance of the amplifier in the actual environmental conditions of the antenna.
This matching method has the considerable advantage, in comparison with older methods, of allowing a matching without iterative computation. A single measurement suffices in order to carry out the matching. It does, however, necessitate a large amount of circuitry and computing means. This circuitry and these computing means prove to be costly and bulky, while in a certain number of applications it is not necessary to use them more than once or a few times.
The object of the invention is to reduce the cost of industrial implementation of automatic impedance matching methods in transmitting and receiving systems which necessitate such a matching operation infrequently.